1. Field of the Invention
The present invention relates to a digital-to-analog converter, and an image display device which includes a function of the digital-to-analog converter in each of driving units for driving respective signal lines of a pixel portion. Here, the digital-to-analog converter includes resistor strings for higher-order bits and lower-order bits of an input digital (video) signal, and samples and holds higher voltage values generated in the higher resistor string, and adds lower voltage values generated in the lower resistor string to the higher voltage value.
2. Description of the Related Art
A horizontal driver for driving respective signal lines corresponding to pixel columns, respectively, is provided in the form of a driving IC in a display panel, for example, a liquid crystal display device.
Although a digital video signal is inputted to the horizontal driver, it is necessary to convert the digital video signal into an analog pixel driving signal. For this reason, a digital-to-analog converter (hereinafter referred to “a DAC” or “a D/A converter”) having a performance corresponding to the number of bits of the video signal (for example, eight bits or more) is built every signal line in the horizontal driver.
Although the various systems have been proposed for the DAC, especially, in an application of an image display device or the like, the DAC needs to be provided so as to correspond to several hundreds to several thousands of video signal lines. Thus, from a request for reduction of an arrangement space, it is necessary to successfully balance a high performance (highly precise conversion) and a small space.
In order to realize the highly precise conversion, a circuit needs to be structured as simply as possible. A resistor string type DAC (D/A converter) is known as a D/A converter using a system meeting this request. This resistor string type D/A converter, for example, is described in Japanese Patent Laid-Open No. 2002-175021.
FIG. 1 shows a basic circuit structure of the resistor string type D/A converter.
A resistor string RS including a series connection body composing of 2N resistor elements RE0, RE1, . . . , RE(2N−2) and RE(2N−1) is connected between an input terminal Tb and an input terminal Tt. Here, a minimum voltage (analog lower limit) Vb of an analog voltage to be outputted is inputted to the input terminal Tb. Also, a maximum value (analog upper limit) Vt of the analog voltage is inputted to the input terminal Tt.
Switches (hereinafter referred to as “select switches”) are connected to nodes between adjacent resistor elements in the series connection body, and a connection node (a connection node on the input terminal Tb side in this example) between an end resistor element and the input terminal Tb or the input terminal Tt, respectively. In the example shown in FIG. 1, a higher select switch S0 is connected to a connection node between the resistor elements RE0 and RE1. Likewise, a select switch S1 is connected to a connection node between the resistor elements RE1 and RE2. Thus, this connection relationship is repeated even for other select switches S3 to S(2N−1) while the resistor elements are shifted one by one.
A side opposite to the resistor elements for 2N select switches S0 to S(2N−1) is short-circuited and is connected to an output terminal To.
When one select switch is selected in accordance with a digital signal, having N bits, inputted thereto in the D/A converter, a desired analog D.C. voltage which is obtained by equally dividing (Vt−Vb) by 2N is outputted in the form of an output voltage Vo to the output terminal To.